1. Field of the Invention
The invention relates to a keyboard device. Particularly, the invention relates to a keyboard device capable of reconstructing a unique clicking sense of an acoustic piano.
2. Description of Related Art
Traditionally, in an electronic keyboard instrument such as an electronic piano, hammers with a specific mass are disposed under the keys, and the hammer may undergo a rotational displacement according to a key-pressing operation. In this way, a specific action load is applied to a key to achieve a sense of key-touching of an acoustic piano. An acoustic piano has a following unique clicking sense, for example, when the key is slowly clicked, a sense of resistance increases in the middle of the process and rapidly decreases thereafter. By reconstructing such clicking sense, the key touching sense of an electronic keyboard instrument can be closer to the key touching sense of an acoustic piano.
Regarding the aforementioned technique, a following technique is disclosed in a Patent document 1 or a Patent document 2. A protrusion is disposed at a specific portion of the hammer, and the protrusion is slidably connected to a chassis (a guide portion) or a key. The technique disclosed in the Patent document 1 or the Patent document 2 is described in detail below with reference to FIG. 7(a), FIG. 7(b) and FIG. 7(c).
FIG. 7(a) is a side view of a keyboard device 501 disclosed in the Patent document 1, which illustrates a key-releasing state, for example an initial state. As shown in FIG. 7(a), the keyboard device 501 includes the following parts: a chassis 502, which is fixed on a shelf of the electronic keyboard instrument (not shown); a key 503, where one end of the key 503 is rotatably fixed to the chassis 502 through a hinge 503a while a pivot 503b serves as a rotation centre, and another end is formed with a flat surface 503c facing downwards; a bump portion 503d with a roughly semicircular cross-section, which is disposed on and protruded from a front portion of the flat surface 503c of the key 503; a hammer assembling portion 502a, which is erected on the chassis 502 under the key 503; a hammer 504, which is rotatably supported by the hammer assembling portion 502a at a rotation shaft 504a as the rotation centre, and is provided with energy to move along a clockwise direction of FIG. 7(a) due to the deadweight of the hammer 504; and a protrusion 504b, which is configured at the back portion of the hammer 504 and leans against the flat surface 503c located behind the bump portion 503d. 
Moreover, in the keyboard device 501, a first key switch 505a and a second key switch 505b used for detecting key pressing information are disposed on the chassis 502. On the other hand, the key 503 has a first switch pressing portion 503e and a second switch pressing portion 503f disposed protruding downwards corresponding to the first key switch 505a and the second key switch 505b. When the key 503 is slowly pressed, the key 503 and the flat surface 503c are displaced in a descending direction, and the first key switch 505a and the second key switch 505b are sequentially pressed by the first switch pressing portion 503e and the second switch pressing portion 503f corresponding to the descending displacement. And, the protrusion 504b of the hammer 504 is in sliding contact with the flat surface 503c, and crosses over the bump portion 503d, and displaces to a front portion (a right side of FIG. 7(a)) of the flat surface 503c. 
FIG. 7(b) is a side view of a keyboard device 601 disclosed in the Patent document 2, which illustrates a key-releasing state, for example an initial state. As shown in FIG. 7(b), the keyboard device 601 includes following parts: a chassis 602, which is fixed on a shelf of the electronic keyboard instrument (not shown); a guide portion 602a (a part of the chassis 602), which is erected at a front portion (the right side of FIG. 7(b)) of the chassis 602; a bump portion 602c with a roughly semicircular cross-section, which is disposed on a flat surface 602b of a back portion of the guide portion 602a in protrusion; a key 603, where one end of the key 603 is rotatably fixed to the chassis 602 through a hinge 603a while a pivot 603b is served as a rotation centre, and another end of the key 603 supports a rotation shaft 604a; a hammer 604, which is rotatably supported by the rotation shaft 604a as the rotation centre, and the rotation shaft 604a is supported by the key 603, and is provided with energy to move along a clockwise direction of FIG. 7(b) due to the deadweight of the hammer 604; and a protrusion 604b, which is configured and protruded at the front portion of the hammer 604, and leans against the flat surface 602b at a top portion of the bump portion 602c on the guide portion 602a. 
Moreover, in the keyboard device 601, a first key switch 605a and a second key switch 605b used for detecting key pressing information are disposed on the chassis 602. On the other hand, the key 603 has a first switch pressing portion 603c and a second switch pressing portion 603d disposed protruding downwards corresponding to the first key switch 605a and the second key switch 605b. When the key 603 is slowly pressed, the key 603 and the hammer 604 are displaced in a descending direction, and the first key switch 605a and the second key switch 605b are sequentially pressed by the first switch pressing portion 603c and the second switch pressing portion 603d corresponding to the descending displacement. And, the protrusion 604b of the hammer 604 is in sliding contact with the flat surface 602b of the guide portion 602a, and crosses over the bump portion 602c, and displaces to a lower portion (a down side of FIG. 7(b)) of the guide portion 602a (the flat surface 602b).
Next, a key load of the keyboard device 501 shown in FIG. 7(a) is described below. FIG. 7(c) is a schematic diagram illustrating a relationship of key stroke and key load. Moreover, in the following descriptions of the keyboard devices 501 and 601, relationships of the flat surfaces 503c, 602b, the bump portions 503d, 602c and the protrusions 504b and 604b are stressed, so that descriptions of the loads produced when the first switch pressing portions 503e, 603c and the second switch pressing portions 503f, 603d press the first key switches 505a, 605a and the second key switches 505b, 605b are omitted.
When the key 503 is pressed, the key 503 rotates downwards along the pivot 503b as the rotation centre. First, a load resulted from an elastic force of the hinge 503a and a provided energy force (the clockwise direction of FIG. 7(a)) of the hammer 504 is applied on the key 503 (a symbol A of FIG. 7(c)). Then, when the protrusion 504b of the hammer 504 is in sliding contact with the flat surface 503c of the key 503, a resistance due to the friction between the flat surface 503c and the protrusion 504b is generated. A load caused by the resistance is gradually increased as the key 503 rotates (a symbol B of FIG. 7(c)). Then, a load caused by a resistance generated when the protrusion 504b of the hammer 504 pushes against the bump portion 503d is applied on the key 503 (a symbol C of FIG. 7(c)), and when the protrusion 504b crosses over the bump portion 503d, the load applied on the key 503 is decreased (a symbol D of FIG. 7(c)).
A key load of the keyboard device 601 shown in FIG. 7(b) is the same, and when the key 603 is pressed, the key 603 rotates downwards along the pivot 603b as the rotation centre. First, a load resulted from an elastic force of the hinge 603a and a mass of the hammer 504 is applied on the key 603 (the symbol A of FIG. 7(c)). Then, when the protrusion 604b of the hammer 604 is in sliding contact with the flat surface 602b of the guide portion 602a, a resistance due to the friction between the flat surface 602b and the protrusion 604b is generated. A load resulted from the resistance is gradually increased as the key 603 rotates (the symbol B of FIG. 7(c)). Then, a load resulted from a resistance generated when the protrusion 604b of the hammer 604 pushed against the bump portion 602c functions on the key 603 (the symbol C of FIG. 7(c)), and when the protrusion 604b crosses over the bump portion 602c, the load applied on the key 603 becomes smaller (the symbol D of FIG. 7(c)).
As described above, in the keyboard devices 501 and 601 disclosed by the Patent document 1 and the Patent document 2, a sense of resistance is increased when the protrusions 504b and 604b push against the bump portions 503d and 602c, and when the protrusions 504b and 604b cross over the bump portions 503d and 602c, the sense of resistance is mitigated.